onExpand：用于捕获节点被展开的事件回调函数，这个是关键。

<ul id="share-to-department-modal-tree"></ul>

var fileTreeSettingForShareToDept = {
	check : {
		enable : true,
		chkboxType : {
			"Y" : "",
			"N" : ""
		}
	},
	view : {
		dblClickExpand : false
	},
	data : {
		simpleData : {
			enable : true
		}
	},
	callback : {
		beforeClick : function(treeId, treeNode) {
			var zTree = $.fn.zTree.getZTreeObj("share-to-department-modal-tree");
			zTree.checkNode(treeNode, !treeNode.checked, null, true);
			return true;
		},
		onCheck : function(e, treeId, treeNode) {

		},
        onExpand: function(event, treeId, treeNode) { //点击展开图标加载子级
     	    var treeObj = $.fn.zTree.getZTreeObj(treeId);
     	    var node = treeObj.getNodeByTId(treeNode.tId);

     	    if (node.children == null || node.children == "undefined") {
     	        $.ajax({
     	            type: 'GET',
     	            dataType: 'json',
     	            url: "/kscd/api/general-storage/share-query-dept/" + treeNode.id + "/",
     	            data: {},
     	            success: function(response) {
     	                if (response.ret) {
     	                    if (response.depts != null && response.depts != "") {
     	                        treeObj.addNodes(node, response.depts);
     	                    }
     	                }
     	            },
     	            error: function(data){
     	            	toastr.error(data.msg, "错误提示");
     	            }
     	        });
     	    }
     	}
	}
};

 初始化时只加载根节点：

$("#share-to-department-button").click(function(){
				    $.ajax({
				        type: "GET",
				        url: '/kscd/api/general-storage/share-query-dept/1/',
				        data: {},
				        dataType: "json",
				        success: function(data){
				        	if(data.ret){
					            $.fn.zTree.init($("#share-to-department-modal-tree"), fileTreeSettingForShareToDept, data.depts);
				        	}else{
				        		toastr.error(data.msg, "错误提示");
				        	}
				        },
				        error: function(data){
				        	toastr.error(data.msg, "错误提示");
				        }
				    });
});